Repair of the lung after injury can restore normal gas exchange or produce a fibroproliferative disorder that leads to excessive scarring and impaired lung function. The goal of this proposal is to elucidate molecular mechanisms that mediate fibrogenesis at the alveolar level. Our hypothesis is that wild-type p53-mediated control of cell proliferation is central to the development of pulmonary fibrosis following inhalation exposure to asbestos and that p53 regulates transcription of PCNA, an essential DNA replication and repair protein, in cells exposed to asbestos. In a rodent model of pulmonary fibrosis, a single inhalation exposure to asbestos fibers initiates the events that generate a fibrotic scar at the sites of fiber deposition in the lung. The preliminary data presented within this proposal demonstrate expression of the p53 tumor suppressor protein at the sites of the developing fibrotic lesions within twenty-four hours of inhalation exposure of rats to asbestos. In most models of fibroproliferative lung disease, the complexity of living systems prevents adequate testing of proposed mechanisms. To characterize p53 function in the rodent model of asbestos-induced pulmonary fibrosis, we will compare cell proliferation and consequent pulmonary pathology of normal and p53- deficient mice after inhalation exposure to fibrogenic asbestos fibers and non-fibrogenic iron particles. p53 functions as a transcription factor that regulates DNA replication, DNA repair and apoptosis. Clearly, the ability of p53 to mediate these functions in a developing fibrotic lesion would have profound effects upon the regenerative capacity of the lung. In agreement with this postulate, our preliminary data indicate that p53-deficient mice resist radiation- induced lung injury. Our observations of lung repair will be correlated with expression of the proliferating cell nuclear antigen, PCNA, an essential DNA replication and repair protein that is also expressed at the sites of fiber deposition, according to our recent observations. Co- expression of p53 and PCNA in the developing lesions is consistent with our preliminary data demonstrating transcriptional regulation of the PCNA promoter by p53. Thus, we propose to test the role of p53 expression directly in an animal model of fibrogenic disease and to elucidate the mechanisms of PCNA promoter regulation in cells exposed to asbestos.